Abstract

The effect of microhydration on the interaction of guanidinium cation with benzene has been studied by employing ab initio calculations. Four different structural arrangements were considered for the guanidinium⋯benzene interaction to which up to six water molecules were added. T-shaped structures are usually the most stable, but as water molecules are included the energy differences with the parallel structures decrease, reaching a point where parallel complexes are even more stable than T-shaped ones. Therefore, the inclusion of water molecules promotes a change in the structure of the cation⋯π contact. The analysis reveals that these stability changes are more related with the structure of the hydrating water molecules than to a modulation of the cation⋯πinteraction. Already with three water molecules, one water molecule in the T-shaped complex has to be located in the second solvation shell, whereas in parallel structures this occurs with four water molecules. As a consequence energy differences among structures decrease. The calculations show that the nature of the interaction is almost unaffected in T-shaped structures, whereas an important dispersion increment is observed in parallel ones, though its overall effect is small.

The authors thank the financial support from the Ministerio de Ciencia e Innovación (Grant No. CTQ2009–12524,) and “Axuda para Consolidación e Estruturación de unidades de investigación competitivas do Sistema Universitario de Galicia, Xunta de Galicia 2007/050, cofinanciada polo FEDER 2007–2013.” We are also thankful to the Centro de Supercomputación de Galicia (CESGA) for the use of their computers.